The present invention relates to a sensor apparatus of the type disclosed in Graef et al. U.S. Pat. No. 4,664,369 for detecting thickness of currency moving along a path in a currency dispensing device. Broadly stated, such apparatus includes a Y-shaped or wishbone-shaped element which is mounted to pivot on a pin or post. The Y-shaped element includes fingers at one end and a tab at the other end, and is mounted on the pin such that the fingers are biased against a plate across which the currency must travel. The tab end of the Y-shaped element includes a metal target which is positioned adjacent a proximity sensor. A set screw is used to adjust the position of the Y-shaped element on the pin to establish a predetermined spacing between the target and the proximity sensor. The proximity sensor acts as a signal generating device and is preferably the type which generates a voltage signal proportional to the distance of the metal target from the sensor.
As currency passes between the fingers of the Y-shaped element and the plate, the Y-shaped element pivots slightly about the pin, thereby displacing the metal target relative to the proximity sensor. Movement of the target relative to the proximity sensor produces a signal indicative of the distance of the sensor from the target. This signal produced by the proximity sensor is characteristic of the thickness of the currency sheet passing between the plate and the fingers.
The arrangement shown in the aforementioned U.S. Pat. No. 4,664,369 has proved to be an extremely successful device for detecting the thickness of currency. One problem associated with such device, however, is that periodic manual re-adjustment of the Y-shaped element is required to reposition the target relative to the proximity sensor, i.e., to center the target within the operating range of the sensor. This adjustment is required because mechanical parts, specifically the fingers of the Y-shaped element which is preferably made of plastic, are worn down by the passing sheets, causing the gap between the proximity sensor and the target to slowly change over time. In this respect, the proximity sensors are very sensitive, having an output voltage change of 0.4 volts for every 0.001 inch gap of change. Consequently, even the slightest wear of mechanical parts produces a noticeable change in the output voltage. This wear typically requires two or three adjustments during the life of the parts. In addition to the cost of a serviceman attending to such adjustments to the system in the field, the currency dispensing device, typically found in an ATM machine, is inoperable until such adjustment is made. In other words, an ATM or like device is out of service until the re-adjustment is made.
The present invention overcomes these and other problems and provides a self-adjusting feedback control circuit for automatically correcting sensor drift caused by mechanical wear.